Electrically conducting polymers have been used in a variety of organic electronic devices, including in the development of electroluminescent (“EL”) devices for use in light emissive displays. With respect to EL devices, such as organic light emitting diodes (OLEDs) containing conducting polymers, such devices generally have the following configuration:anode/buffer layer/EL material/cathodeThe anode is typically any material that is transparent and has the ability to inject holes into the EL material, such as, for example, indium/tin oxide (ITO). The anode is optionally supported on a glass or plastic substrate. EL materials include fluorescent dyes, fluorescent and phosphorescent metal complexes, conjugated polymers, and mixtures thereof. The cathode is typically any material (such as, e.g., Ca or Ba) that has the ability to inject electrons into the EL material.
The buffer layer is typically an electrically conducting polymer and facilitates the injection of holes from the anode into the EL material layer. The buffer layer can also be called a hole-injection layer, a hole transport layer, or may be characterized as part of a bilayer anode. Typical conducting polymers employed as buffer layers include polyaniline and polydioxythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDT). These materials can be prepared by polymerizing aniline or dioxythiophene monomers in aqueous solution in the presence of a water soluble polymeric acid, such as poly(styrenesulfonic acid) (PSS) as described in, for example, U.S. Pat. No. 5,300,575 entitled “Polythiophene dispersions, their production and their use”. A well known PEDT/PSS material is Baytron®-P, commercially available from H. C. Starck, GmbH (Leverkusen, Germany).
Electrically conducting polypyrroles are typically prepared by polymerizing pyrrole or substituted pyrrole monomers in aqueous solution by an oxidative polymerization using an oxidizing agent such as ammonium persulfate (APS), sodium persulfate, potassium persulfate, ferric chloride, or ferric sulfate. The aqueous solution generally contains a water soluble sulfonic acid. In general, enough of the acid is present to function as counter-anions to balance the positively charge polypyrrole backbone, wherein formation of the polypyrrole cation/polymeric acid anion complex renders the polypyrroles electrically conductive. Polypyrrole is available commercially as a 5 wt % aqueous solution from Sigma-Aldrich (St. Louis, Mo.).
The aqueous electrically conductive polymer dispersions synthesized with water soluble polymeric sulfonic acids have undesirable low pH levels. The low pH can contribute to decreased stress life of an EL device containing such material, and contribute to corrosion within the device.
There is a need for improved conductive polymers.